The present invention relates to an image forming method and an apparatus therefor and, more particularly, to a method and an apparatus applicable to a color copier using color toners for adequately controlling the amount of toner deposition on a photoconductive element, or image density. Still more particularly, the present invention is concerned with a method and an apparatus for controlling the bias for development in a color copier.
It is a common practice with a copier or similar image forming apparatus to position a reference pattern having a reference density in part of a platen for preventing the image quality from being degraded by, for example, the contamination of the background. The reference pattern is illuminated to form a corresponding visible reference pattern on a photoconductive element, while a photosensor reads the reference pattern. In the event that a document image is to be formed on the photoconductive element, the charge potential, the bias for development and the amount of exposure are corrected on the basis of the density level sensed by the photosensor. If the apparatus forms the reference pattern while maintaining the bias for development constant, the potential remaining on the element sequentially increases to accelerate toner consumption. Moreover, as the remaining potential of the photoconductive element rises to a given value, the density of the reference pattern reaches saturation to prevent the remaining potential from being accurately detected on the basis of the output of the photosensor. To detect the remaining potential accurately, a reference pattern may be formed by a bias which is corrected on the basis of the density level sensed immediately before, as disclosed in Japanese Patent Laid-Open Publication No. 142370/1988 by way of example.
The above-mentioned implementation, however, has a problem when applied to, for example, a full-color copier using color toners. Specifically, toners used with a full-color copier, especially cyan and yellow toners, cause a great amount of charge to deposit thereon, so that an image density cannot be accurately determined unless the toner density is increased. This is especially true in low-temperature low-humidity environments. When a two-component developer which is a mixture of toner and carrier is used, such a high toner density is apt to smear a developing sleeve. Should the toner be deposited on the developing sleeve, the charge thereof would cause the effective bias for development to deviate to thereby contaminate the entire background, resulting in poor image quality.
It has also been customary to provide the above-described type of apparatus with an implementation for accurately controlling the amount of toner deposition on the photoconductive element, i.e. , the density of a toner image. For example, a photosensor senses not only the density of the reference pattern but also the density of the non-image area of the photoconductive element. Two different densities so sensed are compared. The result of comparison is used to remove an error appearing in the photosensor output due to the scattering in the sensitivity of the photosensor itself, changes in characteristics due to temperature, and contamination or changes in the surface conditions of the photoconductive element, whereby the density of a toner image is maintained constant. This kind of implementation is taught in Japanese Patent Publication No. 14348/1988, for example.
Such a conventional implementation also has a problem when applied to a color copier, especially a color copier of the type using a laser beam. In a color copier using a laser beam, a photoconductive element has therein a layer for diffusing a laser beam so as to eliminate an interference pattern ascribable to multipath reflection. Therefore, most of the reflection from the photoconductive element is only the surface reflection which is weak. As a result, the level of sensed reflection is extremely susceptible to a change in reflectance ascribable to the aging, scratch or similar change in the surface condition of the photoconductive element. In this connection, a traditional photoconductive element implemented with selenium, for example, has an aluminum base which reflects light regularly by more than twenty times than the surface of the element, and the deterioration of surface reflection due to aging is substantially negligible.
Since the reflection from the photoconductive element having the above-mentioned diffusion layer is weak, the detected level is also susceptible to the scattering in the sensitivity and position of a photosensor. It is, therefore, difficult to maintain the density of a toner image constant. Another problem is that since a color toner does not sufficiently absorb infrared rays, the reflection rather increases when the photoconductive element is fully covered with the color toner. In this condition, the photosensor output undesirably has the minimum value, as shown in FIG. 20. As a result, the change in the photosensor output does not match the change in the amount of reflection from the photoconductive element. Regarding the photosensor, therefore, the actual amount of toner deposition is a bilevel function. Simple control which supplies a toner when the output level of the photosensor is higher than a reference value would practically fail to control the toner density when the developing ability is high. To eliminate this problem, the amount of toner deposition on the photoconductive element may be increased to prevent the image density from being lowered. This, however, relies on the operator's perception, i.e. , forces the operator to set a toner density while repeating test copying. This is not only time- and labor-consuming but also increases the number of defective copies.
As stated above, with an image forming apparatus of the type using color toners and a photoconductive element to be scanned by a laser beam, it is not practicable to effect constant control over the toner image density which does not depend on the reflection characteristic of the element.